1. Field of the Invention
The present invention relates generally to a metal film resistor and, in particular, to a method for producing a metal film resistor.
2. Description of the Related Art
Metal film resistors have wide application for configuring circuits in various electric appliances and measuring instruments. For such a metal film resistor, such characteristics as a precision resistor having a wide resistance region ranging from a high resistance to a low resistance as low as 10 m.OMEGA. have recently been required. An excellent temperature coefficient of resistance as well as a small tolerance in electrical resistance value, as small as .+-.1%, is also required.
Conventionally, a metal film resistor employing a copper-nickel alloy film has been known as a resistor which can be applied to usage from a high resistance region to a low resistance region and is excellent in its temperature coefficient of resistance. Such a resistor is generally produced by laminating a multiplicity of copper and nickel films which are alternately and electrolytically deposited on an insulating substrate of porcelain material, and then carrying out a heat-treatment of the laminated films at a high temperature of about 800.degree. C. to form a copper-nickel alloy film.
As an alternative metal film resistor of ultra-precision which is particularly excellent in the temperature coefficient of resistance, there is known a resistor employing a vapor-deposited film of nickel-chromium. The resistor has the characteristics of a temperature coefficient of resistance as small as.+-.10 ppm/.degree.C. and of a tolerance in the resistance value as small as.+-.1%.
The afore-mentioned former copper-nickel alloy film resistors, however, have disadvantage of requiring a heat-treatment at a high temperature, as high as about 800.degree. C. for converting the electrochemically deposited layers of copper and nickel films into a homogeneous alloy. Furthermore, their resistance values are liable to change during the heat-treatment step at such a high temperature, and their manufacturing cost is high because their production requires a large quantity of electric power. In addition, the temperature coefficient of resistance of the obtained resistor is not satisfactory, only showing.+-.250 ppm/.degree.C.
The latter metal film resistors employing the vapor-deposited nickel-chromium alloy film have a disadvantage that the formation of a thick film is difficult because their nickel-chromium film is formed by vapor-deposition. Therefore, in such type resistors, only those having a high resistance in a range from 100 .OMEGA. to 100 k.OMEGA. can be produced.